Gas turbines (“GT”) generally use air bled from one or more stages/ports of the main GT compressor to provide cooling and/or sealing of the components in the path of hot gasses inside the GT. Air is extracted from the compressor and externally routed to the locations that require cooling in the turbine section. Any air compressed in the compressor and not used in generating combustion gases reduces the efficiency of the engine. Therefore, it is desirable to reduce the amount of cooling air bled from the compressor.
This air must have sufficiently higher pressure compared to the pressure of the hot combustion gas inside the GT (back pressure margin), to prevent inadvertent hot gas ingestion into the cooling system. The pressure required by the turbine components determines the stage where the air is extracted from the compressor. To ensure sufficient delivery pressure, it is desirable to select the extraction stage/port with higher pressures. Location of the extraction ports in order to preclude stall and surge is another parameter that limits the available extraction points along the compressor stages. However, extracting air from the earliest possible stage of the compressor will increase the compressor efficiency by reducing the amount of work lost in the extracted air. Therefore, it is desirable to get the cooling flow for turbine components with sufficient back pressure margin using lowest possible stage extraction of the compressor.
Compressors have extraction ports located at different stages to extract air of appropriate pressure for turbine cooling over the entire gas turbine operating conditions. However, sizing the system for meeting the design requirements (for example, minimum flow, backpressure margin, source to sink pressure ratio) at worst operating conditions (i.e., operating load, ambient temperature) leads to excessive compressor bleed on other operating conditions. This leads to loss in both useful power output and efficiency.
FIG. 1 shows the principle of operation of conventional cooling systems in turbomachinery. Compressor 10 has an inlet 16 to draw in ambient air. Compressor air can be extracted from various locations of the compressor and supplied to various locations in the turbine 14 that require cooling. The extraction locations are chosen to supply air at the required pressures. Remaining compressor air 18 is supplied to combustor 12 where it mixes with fuel 20. The hot combustion gas is then supplied to the turbine component 14 via stream 22. A single shaft 34 drives the generator 32. Flow streams 26, 28 and 30 represent cooling air extractions from the compressor that are routed to the turbine section of the turbomachinery for cooling hot gas path components. Streams 26 and 28 supply the low and intermediate pressure coolant, respectively, and they may be routed via external piping to the parts that need cooling. Stream 30 supplies the high pressure coolant and is extracted from a higher stage unit (for example, stage 15, or stage 16 or compressor discharge) in order to meet the back pressure margin as well as the mass flow requirement. Stream 30 is typically routed internally of the turbomachinery, for example, through the bore of the compressor-turbine rotor.
Intermediate and/or lower pressure air is passed through a conventional orifice which regulates the mass flow delivered for cooling, and reduces the excess pressure, before it enters turbine 14, for example, the turbine stage nozzle. However, the static orifice does not adjust to day variations in the ambient temperature. As the variation in the ambient temperature causes variation in the air pressure, this design leads to excess cooling flow extraction and concomitant performance penalty.
As a modification of the above system, typically, a flow-modulating valve is introduced in the path of the intermediate and/or lower pressure air to help regulate the cooling mass flow rate with ambient day variations. However, this does not eliminate the throttling requirement.
A further modification, as explained in U.S. Pat. No. 6,550,253, involves use of an ejector in the intermediate flow path. In this modified system, lower stage flow (for example, 9th stage extraction air) serves as the suction flow and intermediate stage extracted air (for example, from 13th stage) is used as the motive flow. This leads to savings in expensive intermediate stage cooling air and associated compression work. The performance of an ejector is very sensitive to upstream suction pressure as well as discharge pressure variation. For this reason, the performance is affected greatly by ambient day variations.
Priestley (U.S. Pat. No. 6,389,793) discloses an alternate cooling method where an external compressor breathing in ambient air is installed in parallel to the main GT compressor. This increases the availability of air for combustion and therefore it augments GT power output. FIG. 2 shows an example of this alternative method. For the sake of convenience, reference numerals similar to those used in FIG. 1 are used for corresponding components, but with the prefix “1” added. The respective low, intermediate and high pressure cooling air streams 126, 128 and 130 are generated by a separate external compressor 136 driven by motor 138. However, this method requires the external compressor to supply high pressure ratios and hence must be designed accordingly. In addition, the volume flow rates involved are high, thus increasing the external compressor size, weight and, consequently, the cost.
Kozak (U.S. Pat. No. 4,901,520) discloses a cooling system for a GT engine, wherein air is bled from the final compressor, and subsequently is additionally pressurized by a secondary compressor to increase the pressure before it is delivered to the turbine section of the engine. However, in the above cooling system, the secondary compressor is in the interior of the gas turbine, an extension of the first, main compressor. In addition to the extraction pressure being high, the final temperature of the air after the secondary compressor remains high. Hence, there would not be any reduction in the cooling flow requirement and associated non-chargeable air reduction benefits.